Installer/Uninstaller default game location (Uninstaller obsolete)

[bovi]

To clarify, here's the code:

function GetMedFolder(Param: String): String;
var
  MedVersion: String;
begin
  if RegKeyExists(HKEY_LOCAL_MACHINE, 'SOFTWARE\SEGA\Medieval II Total War Kingdoms') then
  begin
        RegQueryStringValue(HKEY_LOCAL_MACHINE, 'SOFTWARE\SEGA\Medieval II Total War', 'AppPath', MedVersion);
        Result := MedVersion;
  end
  else
  begin
    if RegKeyExists(HKEY_LOCAL_MACHINE, 'SOFTWARE\Valve\Steam') then
    begin
      RegQueryStringValue(HKEY_LOCAL_MACHINE, 'SOFTWARE\Valve\Steam', 'InstallPath', MedVersion);
      Result := MedVersion + '\SteamApps\common\medieval ii total war\';
    end
    else
      begin
      if RegKeyExists(HKEY_LOCAL_MACHINE, 'SOFTWARE\Wow6432Node\SEGA\Medieval II Total War') then
      begin
        RegQueryStringValue(HKEY_LOCAL_MACHINE, 'SOFTWARE\Wow6432Node\SEGA\Medieval II Total War', 'InstallPath', MedVersion);
        Result := MedVersion;
      end
      else
      begin
        Result := '[No MII:TW Kingdoms Install Found: You will need to select your "Medieval II Total War" Folder manually]';
      end;
    end;
  end;
end;

Basically the installer reads several places in the registry in sequence to find the location of your M2TW installation, choosing the first one it finds. If it's not found in any of these, give up.
* HKEY_LOCAL_MACHINE\SOFTWARE\SEGA\Medieval II Total War
* HKEY_LOCAL_MACHINE\SOFTWARE\Valve\Steam
* HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\SEGA\Medieval II Total War

So in your case, apparently you have registry entries from your disc install, but are currently using a Steam install instead. Confusion accomplished!